There has been widely spread a liquid crystal display device (liquid crystal display) with a built-in backlight. With such a liquid crystal display, light emitted from a surface illuminant device disposed in the backlight portion penetrates a liquid crystal display element; thereby, an image displayed on the liquid crystal display device becomes visible.
The above surface illuminant device disposed in the backlight portion includes two types, an edge-light surface illuminant device and a direct surface illuminant device. Since the edge-light surface illuminant device is provided with a primary light source disposed in a side face of a light guide plate, it is effective for thinning the surface illuminant device, compared with the direct surface illuminant device. Accordingly, the edge-light surface illuminant device has been widely used as a display portion such as a portable laptop computer and a monitor.
With the edge-light surface illuminant device, a panel made of a transparent resin or the like is used as the light guide plate. This light guide plate includes an incident end face provided with the primary light source such as a linear light source or a dot-light source between an emission plane and a bottom plane facing the emission plane. Moreover, white dots (scattered dots) are printed on the bottom plane, and the brightness distribution in the visual direction is adjusted to be uniformed by adjusting the size, density or the like of the dots. Thereby, the light emitted from the primary light source enters the light guide plate from the incident end face. This incident light is emitted toward the liquid crystal display element portion from the emission plane while being guided in the interior portion of the light guide plate.
Since the above surface illuminant device adopts the scattered dots for the light guide plate, the light right after the emission from the light guide plate includes light distribution diffused at a wide angle, and the light is not directed in the visual direction. Consequently, two prism sheets (directional sheets) which are orthogonal to each other are used for increasing the brightness by condensing the light emitted from the light guide plate in the visual direction.
However, the above conventional edge-light surface illuminant device requires two expensive prism sheets, resulting in an increase in the number of parts for disposing the prism sheets, and causing a problem which complicates the assembling of surface illuminant device.
In order to avoid the increase in the number of parts as described above, there has been proposed a method of directing light to be emitted from an emission plane in the visual direction by forming a prism on the emission plane, a bottom plane facing the emission plane or the like of a light guide plate (for example, reference to patent documents 1-3).
In the patent document 1, for example, there is proposed a light guide plate having an emission plane provided with convex portions (or a groove array of concave portions) which are perpendicular or incline with respect to an incident end face. With such a light guide plate, since the light introduced into the light guide plate is guided to the back of light guide plate, the prism sheets (directional sheet) for bottom laying, which are disposed such that the groove array is arranged in the perpendicular direction with respect to the incident end face, can be omitted. Moreover, according to the patent document 1, there is proposed that the two prism sheets can be omitted by crossing the above convex portions (or the groove array of concave portions).
In this case, the above convex portions (or the groove array of concave portions) can be formed by die making. For instance, the concave portions (or convex portions) each having a predetermined cross section are formed in a cavity plane of metal mold by marking off, and then the cavity plane having a mirror-like smooth and continuous concavity and convexity curved surface can be formed by performing electrochemical polishing.
Moreover, a prism optical element integrated light guide plate is disclosed in the patent documents 2 and 3. Such a prism optical element integrated light guide plate comprises an emission plane and a bottom plane each having V-shape groove array in the direction orthogonal to each other. Thereby, the light entered from an incident end face is introduced into the bottom plane to effectively reflect the reflection light in the emission plane direction. In addition, if the light is emitted from the emission plane via the prism formed on the emission plane, the incident light entered from the direction perpendicular to the incident end face can be emitted at an angle perpendicular to the emission plane or an angle close that angle.                Patent Document 1: JP H09-61631A (FIGS. 6, 8 and paragraph 0038).        Patent Document 2: JP H10-282342A        Patent Document 3: JP 2003-114432A        